Thin Film Etchants
Thin Film Etchants are used to obtain selective etching of gold, nickel, chromium and nichrome films in the photo fabrication of microelectronic circuits. These etchants, used in conjunction with photoresist technology, form precise electrodes and resistor patterns in thin films prepared on alumina substrates.
Thin Film Etchants are room temperature operable solutions. These etchants are compatible with both negative and positive photoresist materials and permit excellent fine line definition.
Thin Film Etch Process
Thin films are prepared on alumina substrate by evaporation or sputtering processes. The film thickness is held uniform for close control of circuit parameters. The following data is typical:
|ELEMENT||SPUTTERING PROCESS||EVAPORATION PROCESS|
|Chromium Resistor||—————————————–||200Å – 1000Å|
|Nickel Conductor||1500Å||5-50 m inches|
|Gold Conductor||5000Å||50-150 m inches|
|Note-gold is often electroplated: Nickel may be electroless plated.|
A photoresist pattern is developed over the entire thin film metallized area. The gold is then selectively etched, followed by nickel etching to generate the conductive elements. The photoresist is removed and a new photoresist pattern developed for the resistors. The resistor elements are formed by etching the chromium or nichrome. Resistance values up to 100 K with conductive terminations are obtainable by the proper sequence of developed photoresist patterns and selective etching. Selection of photoresist and detailed information on the etch process are included in the special instructions.
PROPERTIES OF THIN FILM ETCHANTS
|ETCHANT||ETCH RATE/SEC.||ETCH CAPACITY GM/GAL.|
|Gold, TFA||28 Å (25 °C)||100|
|Nickel, TFB||30 Å (25 °C)||270|
|Electroless Nickel, TFG||50 Å (40 °C)||—|
|Nichrome, TFC *||20 Å (25 °C)||168|
|Nichrome, TFN||50 Å (40 °C)||—|
|Chromium, TFD||20 Å (40 °C)||145|
|Chromium, 1020||40 Å (40 °C)||—|
*1% Sulfuric acid in water should be used as initial rinse, followed by water rinse.
THIN FILM ETCHANTS – TYPE TFA, TFB, TFC, TFD
In microelectronics, it is customary to use a resistor board with three films of metallization: a gold conductor film, a nickel conductor film, and a chromium or nichrome resistive film. These films are deposited on an alumina (Al2O3) ceramic wafer by electroplating, evaporating, sputtering or by electroless methods. The film thickness varies with metallization technique and application. After the proper resist has been applied and an image developed, the microcircuits are produced by selectively etching the resistor board films. Highly selective etching at controlled rates and high compatibility with photoresists are necessary etchant features. It is precisely in these qualities that Transene Thin Film Etchants are most useful.
|FILMS||SPUTTERING PROCESS||EVAPORATIVE PROCESS|
|NICHROME RESISTOR||200 Å||200 Å|
|CHROMIUM RESISTOR||—||200 Å – 1000 Å|
|NICKEL CONDUCTOR||1500 Å||5 – 50 m INCHES|
|GOLD CONDUCTOR||5000 Å||50 – 150 m INCHES|
OUTLINE OF THIN FILM PROCESS
I. Photoresist Process for Conductor Patterns:
- Choose photoresist
- Clean Resistor Board
- Apply photoresist
- Post Bake
- Etch Gold
- Etch Nickel
- Strip off resist
II. Photoresist Process for Resist Patterns
- Apply photoresist
- Post Bake
- Etch Resistor Film
( A) Etch Nichrome
( B ) Etch Chromium
- Strip Off Resist
Thin Film Etch Process
III. Photoresist Process for Conductor Patterns
- Choice of Photo Resist
Recommended photoresists are AZ-111, AZ-1350OH and KMER. However, when etching electroless nickel, AZ-111 or AZ-1350OH are more highly recommended.
- Cleaning of Resistor Board
Immerse substrates in boiling trichloroethylene (transistor grade) for 4 minutes. Force dry with filtered air or nitrogen. Then bake at 110 °C for 30 minutes.
- Application of Photoresist
Apply photoresist by whirl-coating at the appropriate speed.
Bake the photoresist layer at 110 °C for 5 minutes
- Exposing Photoresist
Mask the photoresist layer with conductor pattern and expose for 5-50 seconds to a UV light source such as a carbon arc, mercury vapor or pulsed-xenon lamp. Optimum exposure must be determined by experiment.
Immerse the photo resist layer in the proper developer for the necessary time. Rinse in running water and force dry with filtered air.
- Post Baking
For AZ-111 and AZ-1350OH, post bake at 90 ° – 110 °C for 8 minutes. The KMER photo resist should be post baked at 120 °C for 5-10 minutes.
- Etching Gold
Gold Etchant TFA etches at a rate of 28 Å/sec at 25 °C . Calculate the optimum etching time for the specific thickness of gold film. For example, for a sputtered gold layer of 5000Å, the typical etching time would be 3 minutes. Immerse the board in the etching solution with proper agitation for the correct time. When etching is completed, rinse with running water.
- Etching Nickel
If nickel has been deposited by evaporation or sputtering, the etchant can be used at room temperature. At 25 °C the etching rate is 30Å/sec. Calculate the optimum etching time for the film thickness to be etched. A typical etching time would be one minute. If the nickel has been electroless plated, etching is best accomplished at an elevated temperature of 40 °C and the recommended photoresists are AZ-111 or AZ-1350OH. At 40 °C, the etching rate is 53Å/sec, giving a typical etching time of 4 minutes. Immerse the board in Nickel Etchant TFB for the correct time. After etching is completed, rinse in running water.
- Stripping the Resist
Remove photoresist with proprietary strippers, Force dry the board with filtered air.
- Photoresist Process for Resistor Patterns.
- Recoating of photoresist: Apply photo resist by whirl coating at the appropriate speed.
- Baking: Bake the photoresist layer at 110 °C for 5 minutes.
- Exposing photoresist: Mask the photoresist with resistive pattern and expose to a UV light source.
- Developing: Same
- Post Baking: Same
- Etching the Resistor Film:A. Etching Nichrome
Nichrome Etchant Type TFC, operating at room temperature has an etch rate of 20 Å/sec and a typical etching time of 1/2 minute. Calculate the best etching time for the thickness of the nichrome film. Then immerse the board in the etchant for the correct time. Follow etching with a rinse in running water.B. Etching Chromium (Chromium Oxide)
Calculate the necessary etching time for the specific film thickness of chromium.
Etching time of one minute is typical at an elevated temperature of 40 °C with an etching rate of 40Å/sec. Follow etching with rinse in running water.
- Stripping The Resist
Remove photo resist with proprietary strippers.
Properties of Thin Film Etchants
|Thin Film||Etchant||Etch Rate
|Recommended Resist||Typical Etching Time|
|Gold||Gold Etch TFA||28 Å/sec||—||AZ-111 AZ-1350OH KMER||3 MIN|
|Nickel evaporated or sputtered||Nickel Etch TFB||30 Å/sec||—||AZ-111 AZ-1350OH KMER||1 MIN|
|Nickel Electroless||Nickel Etchant TFB||—||53 Å/sec||AZ-111 AZ-1350OH KMER||4 MIN>|
|Chromium Cr (Cr2O3)||Chromium Etchant TFD||—||40 Å/sec||AZ-111 AZ-1350OH KMER||1 MIN.|
|Nichrome||Nichrome Etchant Type TFC||20Å/sec||—||AZ-111 AZ-1350OH KMER||1/2 MIN.|